1. Field of the Invention
The present invention relates to a power line communication (PLC) system for performing communications using power lines and to a capacitive signal coupling unit (also known as a capacitive signal coupler) used in such a PLC communication system.
2. Description of the Related Art
Power line communications are communication techniques in which signals are superimposed on power lines such that the signals are sent and received via the power lines. In recent years, carrier waves of high frequencies have tended to be used to carry signals in PLC rather than carrier waves of low frequencies, and high-speed, high-capacity communications have been accomplished. Therefore, a technique combining two modes of signal transmission is under discussion. In particular, signals are transferred to electric power substations or power delivery transformers by optical cables or other communication technique. The signals are then transferred to individual homes by communications making use of power lines included in overhead lines or underground lines.
In order to superimpose a signal onto a power line, it is necessary to electrically connect the power line and a communication device. Because the power line is supplied with electric power, it is impossible to directly connect the communication device with the power line of a voltage, for example, of the order of kV to tens of kV. Therefore, a signal coupler for injecting a signal to the power line or extracting a signal from the power line is mounted to the power line. The communication device is connected with the signal coupler. Consequently, communications between communication devices are enabled via the power line.
Intrinsically, a power line is not designed to transmit high-frequency signals. Therefore, where a high-frequency signal is transmitted through the power line, electromagnetic waves leaking from the power line present problems. In one known technique (for example, see the following patent reference 1), a clamped coil is inserted into the grounded line to enhance the degree of balance of the power line. The coil reduces the common current flowing through the power line, thus decreasing the leakage of electromagnetic waves.
Patent reference 1: JP-A-2004-336456 (page 3, lines 45-48 and FIG. 1)
It has become necessary to pay attention to electromagnetic waves leaking from the vicinity of a capacitive signal coupling unit in a desired frequency range, as well as to electromagnetic waves leaking from high-frequency signals transmitted through the power line, for the safety of the human body and because of noise problems with devices caused by electromagnetic waves.